Polypropylene homopolymers are widely used in packaging applications. Polypropylene homopolymers show balanced properties and are cheap, easily decomposable polymers, but they show poor transparency and low impact strength.
It has been suggested to improve the poor optical properties and the impact strength of polypropylene homopolymers by adding nucleating agents and/or clarifiers. Sutiable nucleating agents are for example talcum, disorbitol, organic phosphates and the like.
Nevertheless the desired properties of propylene homopolymers cannot be improved to the extent desired, namely to show high impact strength at low temperatures.
Compositions with improved impact strength and improved optical properties are for example propylene/α-olefin random copolymers, wherein the α-olefin is ethylene or an α-olefin with 4-10 carbon atoms. Although transparency is higher and impact strength is improved considerably at ambient temperature, impact strength at lower temperature, i.e. 0° C. and below is still not satisfying. Propylene/α-olefin random copolymers have a ductile to brittle transition temperature between −5° C. and 5° C. depending on the monomer content and therefore impact strength at lower temperatures is still very poor.
To improve the impact strength at lower temperatures heterophasic polymer compositions have been suggested. Heterophasic polymers are polymers having a matrix phase (phase 1) and a second phase. The matrix phase is usually a polypropylene homopolymer or polypropylene/α-olefin copolymer phase and the second phase is usually a propylene/α-olefin rubber polymer.
The polymers of the second phase have low glass transition temperatures, usually below −30° C. Therefore impact strength of such heterophasic systems is rather satisfying, even for deep freezing applications. A major drawback is the poor transparency due mostly due to big rubber particles in the heterophasic system.
Several attempts have been made to improve the transparency of heterophasic polymer systems.
EP-B 0 373 660 discloses heterophasic polymer systems with improved impact strength and improved transparency an elastomeric propylene/ethylene as matrix phase and a phase 2 being propylene/α-olefin rubber, wherein the intrinsic viscosity ratio between phase 1 and phase 2 has to be a defined value, namely 0.2. Therefore the heterophasic system of the disclosure of EP B 0 373 660 is restricted to very special combinations of phase 1 and 2 polymers.
EP-A 0 814 127 discloses embrittlement resistant polyolefin compositions and flexible articles thereof based on propylene homopolymers or propylene/α-olefin copolymers and elastomeric copolymers of ethylene with propylene or butene-1. The elastomeric copoylmeres enhance the flexibility without affecting the clarity of the polymer composition, which is not at least satisfying for packaging applications, although these compositions are claimed to be used for the production of medical articles and food packaging material.
EP B 0 593 221 discloses polypropylene resin compositions comprising polypropylenes of defined MFR and ethylene/α-olefin copolymers with good balance between impact strength and rigidity and high moldability useful for automotive parts. These compositions do not have good optical properties, especially low transparency and high haze.